1. Field of the Invention
The present invention relates to a switching regulator used as a power source for an industrial electronic machine and the like, and more particularly, to a voltage drop detecting circuit for detecting a drop in an output voltage and issuing a voltage drop warning by outputting a signal to the outside.
2. Description of the Related Art
Switching regulators, in contrast with other regulators, are small-sized, light in weight, and highly efficient, and can cope with a wide range of input voltages, and thus are currently widely used as power sources for industrial electronic machines, such as computers, and servomotors, etc. Nevertheless, an erroneous operation of these industrial electronic machines may occur or the components thereof may be damaged if a drop in the supply voltage due to a malfunction of the power supply is not corrected. Therefore, a voltage drop detection circuit is provided on the power-supply side, separately from a voltage stabilizing circuit, to thereby improve the reliability of the whole system using the power supply. Namely, if a drop in the voltage occurs, the voltage drop detection circuit outputs a signal to the outside, to thereby quickly give an alarm or cut off the power supply.
FIG. 4 shows a circuit diagram of a forward-type switching regulator having a conventional voltage drop detecting circuit. In FIG. 4, A1 and A2 designate input terminals, which are connected to a DC power source, and B1 and B2 designate output terminals to which a stabilized DC voltage is delivered.
The voltage drop detection circuit is denoted as 5 and includes a comparator IC2, which compares the output voltage with a reference voltage of a Zener diode ZD1, and delivers a signal to terminals G1 and G2 when a drop in the output voltage occurs. H1 and H2 designate power supply terminals for the comparator IC2, and are connected to a DC power source.
The comparator IC2, however, cannot operate normally unless constantly supplied with a stabilized voltage, and thus the output of the switching regulator to be detected cannot be used as the power source for the comparator IC2. Namely, a separate power source to ensure a stabilized voltage is required. Moreover, a reference voltage source, such as the Zener diode ZD1, must be provided to set the detection level.